In the provision of realistic joints, as used in prosthetic limbs, an important aspect in attempting to achieve a realistic movement is to provide a different operating characteristic to the joint when under load. Indeed, one of the more important characteristics of an artificial leg for achieving a natural-looking walking gait correspond with those of a so called (friction-) stabilised knee, i.e. a knee which resists flexion when under load, that is when it is bearing at least some of the weight of the user, for example, an amputee. As an almost default prescription item in the UK, a sudden fall or collapse in the use of an artificial limb can provide a nasty surprise. Indeed, it will be appreciated that such types of fall are a serious issue for the healthcare providers and patients alike. Other mechanical knee joints are also be liable to collapse in particular situations. Reference is made to literature associated with the Advanced Prosthetics course by J. Boender at Strathclyde University, which provides a detailed review of artificial knee joints.
Early prosthetic limb systems, dating from the 1950's were provided with friction brake devices. For example, in GB779087, when utilized in a knee joint application, there was a provided a shin and knee joint mechanism which included a drum fixed to the shin, with one or more bands connected to the thigh and embracing the drum so that the bands gripped the drum to lock the knee when the leg was bearing weight, actuating means associated with the shin and thigh operated to release the lock just before the foot left the ground in walking, with a connection between the shin and thigh permitting relative axial movement between the shin and thigh. In this device, however, an axial load on the limb produced a small rotation of the radius arm or arms causing the brake band or brake shoe to grip the drum and to resist knee flexion. Indeed, the resistance would frequently become so great such that the knee became automatically locked once sufficient load had been applied. Later devices were combined with a pneumatic piston and cylinder assembly which applied lower degrees of resistance to flexion and/or extension of the knee to control the motion of the shin during the swing phase.
In recent years, however, such friction-based systems—which required regular servicing and adjustment have been replaced by hydraulic dampers with external control, which provide resistance to flexion during a stance phase as well as a swing phase of operation by means of a piston and cylinder assembly. Hydraulic artificial knees provide stability to the prosthesis when the patient's weight is borne on the prosthesis, and collapse must be prevented. To prevent collapse of a free artificial knee joint the joint must receive appropriate information to inform it of its required mode of function. The hydraulic knee joint operates by utilising a volume of incompressible fluid to the knee joint, whereby to provide mechanical stability.
One example of such an arrangement is the hydraulic “S-N-S” knee control system manufactured by Mauch Laboratories, Inc. In some situations, however, this system required an amputee to make a knee-extending movement before flexion could be initiated. Additional problems arose through external wear and through the fact that they require actuation, which is, of course dependent upon movement being regular. As is known, when walking, one will vary one's gait to go down stairs, to cross steps, to avoid obstructions and the like. In some cases mechanical switching of the valve will not be effected properly. U.S. Pat. No. 5,376,137 to Blatchford is an example of a weight activated knee joint with hydraulic amplification of weight application triggered pivotal movement, whilst U.S. Pat. No. 6,106,560 to Ultimate Knee teaches of a weight activated knee joint with mechanical amplification of weight application triggered pivotal movement.
Whilst recent hydraulic devices are believed to be much improved they are complex and costly to manufacture; they are manufactured to high tolerances. If mechanical external valve control is provided, then there will be problems as discussed above. Alternatively, electronic control and flow control valves can be provided—that are expensive to purchase and maintain—which enable amputees to walk with a pre-determined gait yet will not necessarily be appropriately reactive to uneven surfaces.
Despite the above advances, the likelihood of buckling in an artificial knee joint in a mechanical external prosthesis remains a concern. In such devices it has proven to be possible to use the weight of the patient as a means to energise a stabilising mechanism on heel strike, whether by way of a friction brake clamp or a closure of a hydraulic valve. However, the mechanisms of such devices provide a reduced benefit when the knee joint is not extended fully upon heel strike upon a supporting surface, giving rise to a severe risk of the collapse of the transfemoral prosthesis. Examples of such prostheses are known under the following trade names: 3R80 produced by Otto Bock, Ultimate Knee produced by Ortho-Europe, Total Knee produced by Ossur. Additionally, these types of prosthesis are known from, for example, GB779087 and U.S. Pat. No. 5,376,137.
The same threshold also makes it difficult to find instant effortless stability of the knee, when extension after mid swing is incomplete the respective foot is susceptible of hitting the ground too early. This is particularly true when traversing rough, littered or overgrown pathways. Typically the weight-activation class of knee joints do not provide security against collapse in such conditions or in the event of accidental use.
To overcome this disadvantage, other devices have been made the keep the knee joint in a default state to accept weight on heel strike, and on a hyperextension of the knee joint the knee device is permitted to allow movement and release the low resistance swing mode. In this class of device, when a user faces a steep slope to walk down, an involuntary hyperextension effort is exerted on the knee joint out of necessity. An amputated femoral end comprises a fleshy rather than boney termination to the femur, which presents problems as it sits loose in the socket with which it seeks to be securely retained. In seeking to grip with the socket or the well of the prosthesis from inside, and in preparation for movement of the hip musculature, control of the descent of the slope is enabled whilst using the ‘yielding’ mode of such a knee joint. Accordingly, there is a significant risk of releasing the swing mode of the prosthesis whilst expecting a yielding stance mode when collapse is imminent. An example of a presently available prosthesis which displays such a characteristic is the Mauch SNS (Ossur).
Problems arising from the use of weight activated knee joint control mechanisms include the fact that the residual weight taken by the artificial limb on toe off can be inhibitive to the release of the weight-activated mechanism. Typically an apparatus or means is supplied to cause a threshold value of weight required to activate the knee stability. This threshold is easily overcome by force acting through the heel and is not easily overcome by force acting through the toe. Nevertheless, this threshold takes away from the ease of activation on heel strike, which subtracts from the total ease of use of the artificial limb, and a minimal level of attention is required in use, although a patient will become accustomed to this and whilst the movement becomes a natural reaction in due course, it will induce an unnatural swagger, visible to onlookers and, moreover, in a lapse of concentration, when tired, for example, may cause a lapse in behaviour of the limb with a resultant fall.
An electronic solution has been found in the C-leg (Otto-Bock), wherein strain gauges are used to inform an onboard electronic algorithm about the state of the joint, such that swing release will only be permitted when a load is arising from the toe (i.e. the leg must be end of stance phase), and the knee must be straight (i.e. the leg must be end of stance phase). A load vector through the toe that does not pass anterior to the knee joint will not release the knee joint into swing. Similarly, a load that does not pass through the toe, but passes anterior the knee joint will not allow the joint to engage a swing phase mode. Whereas this solution stands out in clinical performance, these devices are too costly to be available for the common user. Moreover, it is counter intuitive to place one's body weight onto the device to secure the same body weight against sudden collapse. Thus, whilst this computerised and thus expensive prosthesis can provide a lock-out solution, since the solution is counterintuitive; when utilised it does not provide a suitable degree of comfort for the user.